1. Field of the Invention
The present invention relates to an EGR valve device installed in an exhaust-gas recirculation passage of, e.g., a diesel engine, and more particularly to an EGR valve device effective for prevention of leakage of exhaust gas when the exhaust-gas recirculation passage is closed.
2. Description of the Related Art
As a conventional EGR valve, a double-poppet type EGR valve device is well known which includes a valve housing having one or more exhaust-gas inlet ports (hereinafter referred to simply as an inlet port) and two or more exhaust-gas outlet ports (hereinafter referred to simply as an outlet port), each being connectable to the exhaust-gas recirculation passage of an engine, and forming a primary passage located on the inlet port side and a secondary passage branching out from the primary passage toward the outlet ports; a first and second valve-sheets disposed in the branched communicating portion between the primary passage and the secondary passage; a valve shaft axially movably assembled within the valve housing; and first and second valves secured on the valve shaft, and almost simultaneously abut on the first and second valve-sheets, respectively, when the valve shaft moves in one direction, to close the exhaust-gas recirculation passage. Such double-poppet type EGR valve devices are classified into largely two types: a built-in type installed in the exhaust-gas recirculation passage where the entire valve device is being exposed to the air, and a drop-in type in which part or most of the valve housing is assembled inside the recirculation passage.
The operation of the traditional EGR valve device will now be described below.
In both of the EGR valve devices of a built-in type and of a drop-in type, since one or more inlet ports and two or more outlet ports thereof are connected to the exhaust-gas recirculation passage, both of the valve housing and the valve shaft positioned within the valve housing are heated by high-temperature exhaust gas circulating through the exhaust-gas recirculation passage. However, since, as discussed in the above, the built-in type EGR valve device is assembled into the recirculation passage where the entire valve housing thereof is being exposed to the air, the valve housing is kept in the state of being always cooled down by the air, and since the valve shaft is placed within the valve housing, and exposed to high-temperature circulating exhaust gas in such a condition where the valve shaft is being shut off from the air, a temperature difference is engendered as a necessary consequence between the valve housing and the valve shaft both heated by the circulation exhaust gas. Due to this temperature difference, the valve housing and the valve shaft have different elongation percentage in the axial direction caused by their respective thermal expansions. This creates inconsistency of the distance between the two valve-sheets integrally provided within the valve housing and that between the two valves integrally secured on the valve shaft. Consequently, although the two valves are arranged with the intension of closing them simultaneously, only one valve is allowed to seat on the valve-sheet, and a gap formed between the other valve and valve-sheet widens as a temperature of the circulating exhaust gas goes up, resulting in an increase of leakage of the exhaust gas therefrom.
Moreover, even in the drop-in type EGR valve device, because the periphery of the valve housing disposed within the exhaust-gas recirculation passage is partially contacted with the exhaust-gas recirculation passage through a sealant, and the exhaust-gas recirculation passage is exposed to the air, heat of the valve housing heated by the high-temperature exhaust gas circulating through the exhaust-gas recirculation passage is propagated to the exhaust-gas recirculation passage which is being cooled down by the air. This brings about a temperature difference between the valve housing and the valve shaft. Accordingly, as with the built-in type EGR valve device, the valve housing and the valve shaft have different elongation percentage in the axial direction caused by their respective thermal expansions, and thereby only one valve is permitted to seat on the valve-sheet. The gap formed between the other valve and valve-sheet widens with rising temperature of the circulating exhaust gas, leading to increased leakage of the exhaust gas.
To this end, EGR valve devices have also been proposed in which countermeasures are taken for reducing leakage of the circulating exhaust gas caused by the difference in the elongation percentage resulting from the above-described temperature difference between the valve housing and the valve shaft. Giving an instance as one countermeasure, U.S. Pat. No. 6,247,461 B1 discloses an EGR valve device arranged such that a thermal expansion coefficient of the valve housing member located between at least two valves is equal to that of the valve shaft. Further, as another countermeasure, JP 11-182355 A (Page 7 and FIG. 2) discloses an EGR valve device arranged such that two valves do not simultaneously abut on two valve-sheets when the valves are fully closed in an ordinary temperature atmosphere, but they almost simultaneously abut on the two valve-sheets, respectively when the valves are fully closed in a high temperature atmosphere. In other words, JP 11-182355 A discloses an EGR valve device arranged such that differentiating the distance between the two valves and that between the two valve-sheets when the valves are fully closed in a predetermined temperature, a clearance is formed between one valve and one valve-sheet under the condition that the other valve abutted on the other valve-sheet.
Since the conventional EGR valve devices have been arranged as mentioned above, a temperature difference engendered between the valve housing and the valve shaft, even if it is configured that the valve housing member located between at least the two valves is set so as to have the same expansion coefficient as that of the valve shaft, as disclosed by U.S. Pat. No. 6,247,461 B1. The temperature difference creates inconsistency of the distance between the two valves incorporated on the valve shaft and that between the two valve-sheets incorporated within the valve housing. As a result, the gap, which formed between one valve and one valve-sheet under the condition that the other valve abutted on the other valve-sheet when the valves are fully closed, goes beyond the tolerance. Consequently, the higher the exhaust gas temperature, the more and more the phenomena tends to become conspicuous with the result that leakage of the circulating exhaust gas increases.
Moreover, as disclosed in the above JP 11-182355 A, when it is arranged that the two valves abut on their respective valve-sheets under the condition that the valves are fully closed in a high temperature atmosphere in which the high-temperature exhaust gas is circulating through the exhaust-gas recirculation passage, a clearance formed between one valve and one valve-sheet under the condition that the other valve seated on the other valve-sheet at the time of fully closing the valves at the normal temperature. As a result, a large amount of circulating exhaust gas leaks from the clearance.